1. Field of the Invention
The present invention relates to an electrophotographically sensitive material for a litho printing plate. More particularly, the present invention relates to an improved electrophotographic litho printing plate material which is capable of being connected to a printing plate having a high resistance to deterioration in quality of printed images with the lapse of time, a reduced elongation, and a high printing durability.
2. Description of the Related Art.
Due to the development of small size offset printing machines and automation devices for such printing machines, the mainstream of light printing processes is concentrated on the offset printing method.
In the printing plate materials for the offset printing process, many approaches in research and development were made and various new materials were provided and have been practially utilized.
The offset printing plate material usually comprises a substrate sheet and a photoconductive layer provided on the substrate sheet. Especially, it is known that an electrophotographic litho printing plate material having a photoconductive layer containing, as a principal component of a photoconductive pigment, zinc oxide (ZnO) particles, is cheap and is easy to use in the preparation of the printing plate. Therefore, the mainstream of the offset printing plate is concentrated on the above-mentioned type of electrophotographic litho printing plate.
In the preparation of the electrophotographic litho printing plate, a photoconductive layer of a corresponding litho printing plate material is coronacharged, exposed to actinic radiation, and developed in a desired pattern of images, and the developed images fixed by using a platemaker.
Generally, the developing procedure is carried out by a dry-developing method wherein a mixture of a toner with a carrier consisting of, for example, iron powder is used as a carrier, or by a wet-developing method wherein a developing liquid consisting of a toner dispersed in an organic solvent, for example, is used.
The wet-developing method is advantageous in that the reproductivity of resultant images in an intermediate tone is excellent, the resolving power is excellent, the platemaking time is short, and no correcting procedure is necessary. Therefore, in preparation of the electrophotographic litho printing plate, the wet-developing method is most widely used.
The electrophotographic litho printing plate material must be not only adequate for the above-mentioned printing plate-producing method, and the resultant images exhibit a high quality, but also the resultant printing plate must exhibit an excellent printing property.
For example, the litho printing plate produced by the above-mentioned developing procedure must be provided with non-image portions which are capable of being fat-desensitized. Also, in the electrophotographic litho printing plate, when the photoconductive layer surface which has been exposed to an actinic radiation and then developed is treated with an etching liquid, the non-image portions of the developed photoconductive layer must be capable of being made hydrophilic. Furthermore, as the printing plate surface is brought into contact with a large amount of water during the printing procedure, the plating plate must have a high resistance to water.
In consideration of the above-mentioned requirements, it has been attempted to provide an intermediate layer between the substrate sheet and the photoconductive layer to enhance the water-resistance of the printing plate.
Generally, the intermediate layer of the electrophotographic litho printing plate material is required to provide the following properties.
(1) high barrier property
(2) excellent resistance to solvent for photoconductive layer,
(3) excellent resistance to water,
(4) proper electric resistivity of from 1.times.10.sup.9 to 9.times.10.sup.11 .OMEGA.-cm at a temperature of 20.degree. C. at a relative humidity (RH) of 65%.
Usually, a high organic solvent resistance causes the printing plate to exhibit a poor water resistance, and a high water resistance causes the printing plate to exhibit a poor organic solvent resistance. Therefore, it is necessary that the printing plate exhibit a well balanced and proper water resistance and organic solvent resistance. To attain this balance, there are various approaches. For example, a conventional intermediate layer in the electrophotographic litho printing plate material may contain a water-proofing agent and a matrix binder. The matrix binder usually consists essentially of a water-soluble polymer, for example, polyvinyl alcohol, casein or starch. The water-proofing agent consists, for example, of a polyamino resin. The water-soluble polymer may be made water-insoluble by cross-linking the water-soluble polymer with the polyamino resin through a cross-linking agent. The matrix binder may be formed from a mixture of an aqueous solution of the above-mentioned watersoluble polymer and an aqueous emulsion of a synthetic water-insoluble polymeric material.
When the cross-linked polymer material is used to provide the intermediate layer, if the cross-linking reaction is carried out in an intermediate layer-forming paint liquid before the paint liquid is applied to a substrate, the viscosity of the paint liquid is increased and then the liquid is gelled, so that the paint liquid becomes useless for practical application. If it is attempted to carry out the cross-linking reaction during or after the application of the paint liquid to the substrate, it is necessary to heat the resultant paint liquid layer at a temperature of 150.degree. C. or more for 5 minutes or more. These heating conditions are, however, not applicable to an actual process for producing the intermediate layer, in consideration of the productivity and commercial profit of the process.
Also, if above-mentioned high temperature and the above-mentioned long time are applied, the substrate will be deteriorated and, therefore, it becomes impossible to obtain a litho printing plate having the necessary mechanical and physical strength.
That is, the heating temperature and time expected from usual coating conditions are unsatisfactory to complete the cross-linking reaction. Therefore, the resultant intermediate layer exhibits an unsatisfactory water-proofing property, and then the resultant litho printing plate exhibits an unsatisfactory durability for the printing procedure.
Also, when a hydrophilic substance is contained in an intermediate layer of an electrophotographic litho printing plate, the resultant intermediate layer exhibits a poor bonding property to a photoconductive layer containing a hydrophobic bonding material.
Japanese Examined Patent Publication No. 47-47610 for Ricoh Co. discloses an intermediate layer-forming paint liquid consisting of a polyvinyl resin containing a functional monomer, for example, hydroxyethyl acrylate, and an amino resin. In this type of paint liquid, the reaction of the functional groups in the polyvinyl resin with the amino resin is carried out slowly, the resultant printing plate exhibits a poor water-proofing property and a poor decay of light in the image-forming procedure, and therefore, the resultant images are spoiled by serious blushing and poor contrast.
Also, Japanese Examined Patent Publication No. 40-7332 for Iwasaki Tsushinki discloses a printing plate in which a surface of a substrate is coated with a hydrophilic polymeric material layer and a coating layer, consisting of a synthetic polymeric resin soaked or swollen with a solvent for photoconductive coating layer, is formed between the hydrophilic polymeric material coating layer and the photoconductive coating layer.
In this type of printing plate, the added step for forming the hydrophobic coating layer between the hydrophilic polymer material coating layer and the photoconductive coating layer increases the cost of the resultant printing plate and the hydrophobic coating layer exhibits a bad influence on the decay of light on the photoconductive coating layer.
Japanese Examined Patent Publication No. 48-11902 for Ricoh Co. discloses an intermediate layer-forming paint liquid containing a synthetic resin emulsion, a hydrophilic polymer material and a water-proofing agent, for example, glyoxal, urea-formaldehyde resin, and melamine-formaldehyde resin.
This type of intermediate layer exhibits an unsatisfactory water-proofing property. Also, the intermediate layer is disadvantageous in that glyoxal or formaldehyde in the water-proofing agent is isolated from the intermediate layer with the lapse of time, the isolated glyoxal or formaldehyde is absorbed by zinc oxide particle surfaces in the photoconductive coating layer so that the photoconductive coating layer is degraded, and this phenomenon causes the photoconductive coating layer to exhibit a decreased oxygen-reabsorption after radiation exposure, and a degraded exposure property.
Japanese Unexamined Patent Publication No. 50-96231 for Canon Co. discloses a photoconductive coating layer containing a silane coupling agent. This is to enhance the durability in the printing process of the resultant printing plate by improving the photosensitivity of a duplicating photosensitive material and the bonding property of a photoconductive coating layer and a substrate. However, it was not at all known before the present invention to add a silane coupling agent to an intermediate coating layer.